I think that automatic fire is dependent upon good logistics that enables the front line soldiers to be continually resupplied with ammunition to the fill the air with lead. I am thinking about the first paratroopers that dropped behind enemy lines in Normandy on D Day. They had only so much ammunition and if they didn't link up with any supplies- that was it. I don't know how the army fights these days but years ago everyone had an assigned area and was supposed to hit any enemy in their area. You hopefully could set up a cross fire situation. You could have bolt actions rifles and just a couple of automatic weapons to take care of any sudden charge at your position. In any event, ammunition was a valuable item and I think the traditional view was that it shouldn't be wasted.